hmm im personally really really high on Calvin Johnsons... but Rivers so far has been a beast, who is your starter that you'd be able to get rid of him so easily?
Do it, especially if you already played the guy. I would try and get something else from him though, if only a handcuffed RB.
I'm a big believer in upgrading my starters by using unneeded back-ups so you should probably do it, but the trade is a bit slanted in his favor so I would try to get something else form his end.